A special type of small granule-containing cell in the abdominal para aortic region of the frog.

Light and electron microscopic studies have been made of a special type of small granule-containing cell (termed Type IV cell) in the frog abdominal para aortic region. These cells contain numerous dense granular vesicles (100--150 nm in diameter) and are considerably smaller (10--20 mu) than neighbouring nerve cells, although they have many features in common with them. They do not resemble chromaffin cells as do Types I, II and III cells. The cell bodies are completely ensheathed by satellite cells and are isolated from neighbouring cells of the same type. Type IV cells have long processes which usually become incorporated in bundles containing 2--20 processes, including some cholinergic nerve fibres, and are loosely enveloped by perineurium. The termination of the processes of Type IV cells do not appear to form efferent synapses on nerve cells at least within the para aortic region or in paravertebral sympathetic ganglia. A close topographical relationship is not found between these processes and blood vessels. It is suggested that the small Type IV granule-containing cells in the frog abdominal para aortic region are not interneurons or neurosecretory cells, but are a special type of sympathetic nerve cell.